Integral nuclear reactor plants generally have an annular space between the central reactor core and the reactor shell. Positioned within this annular space are the steam generating tube bundles across which heating fluid from the reactor core is to be passed. Pumping apparatus is normally positioned below the annular space and reactor coolant fluid, after passing across the tube bundles, is arranged to be drawn into this apparatus.
Generally, a number of tube bundles, each occupying the same amount of space, are positioned in the annular space. Owing to the limited space available for the tube bundles it is difficult to locate a sufficient number of tubes in the annular passage and to make the tubes of sufficient length to allow production of superheated steam.